This invention relates in general to blood pressure measuring devices and in particular to such devices which are automatic and do not require a stethoscope for identifying systolic and diastolic conditions and which may also provide mean pressure and heart rate in a direct reading manner.
Blood pressure measurement as a medical procedure is not new nor are the basic concepts or equipment which are employed new. Conventional measurement procedures require that a compression cuff be snugly wrapped around the patient's right arm approximately one inch above the antecubital fossa location. The compression cuff includes an inflatable bladder at one end and this bladder must be applied directly over the artery with the remainder of the compression cuff being used as a wrap to secure the bladder in place. The compression cuff (actually the inflatable bladder) is inflated rapidly to a pressure above the assumed or previously determined systolic pressure. By means of a manually released valve, the cuff is deflated at a rate of two to three millimeters (mm) of mercury (Hg) per heartbeat. With the bell of a stethoscope placed snugly over the artery (thereby producing an airtight seal) the physician may either watch an aneroid dial or column of Hg of a suitable sphygmomanometer and note the pressure at which characteristic changes in the Korotkoff sounds occur. From these sounds, created by pressure controlled compression waves, the systolic and diastolic blood pressures can be determined.
This conventional method has various shortcomings in terms of convenience, versatility and accuracy. It is necessary for the physician or medical personnel taking the blood pressure to accurately position the inflatable bladder over the artery, to make an air tight seal with the bell of a stethoscope over the artery, to manually control the pressure release rate and to attempt to read the corresponding pressure when particular characteristic sounds are detected through the stethoscope.
In an attempt to improve blood pressure measuring techniques, certain modifications have been made to conventional blood pressure measuring equipment. Examples of some of these modifications are shown by the following patents:
______________________________________ Patent No. Patentee Issue Date ______________________________________ 3,651,798 Egli et al. 3/28/72 3,993,047 Peek 11/23/76 4,005,701 Aisenberg et al. 2/01/77 ______________________________________
Egli et al. discloses an electronic blood pressure indicator for use with conventional blood pressure measuring equipment. This device includes a microphone for translation of arterial blood flow sounds into electrical signals. The electronics are somewhat complicated due to the need to filter out undesired and extraneous frequencies, and to distinguish the characteristic changes in the Korotkoff sounds.
Peek discloses a type of electronic blood monitoring device which is used to monitor blood circulation conditions. This device includes an infared radiation source and a corresponding sensor which are placed in contact with the skin of the subject. This device is not related to conventional sphygmomanometers nor is it usable for reading blood pressure.
The patent to Aisenberg et al. discloses a noise-rejecting electronic sphygmomanometer which utilizes two microphones positioned adjacent the inflatable bladder in a conventional compression cuff. The first microphone is used to pick up Korotkoff sounds as the cuff is deflated and blood flow in the patient's arm resumes. The second microphone is used to pick up background noises so that electronic circuit discrimination and signal comparisons can be used to provide an accurate sound indication which is unaffected by such background noises.
With each of the aforementioned devices there are numerous shortcomings which exist. To begin with, all presently available blood pressure measuring devices require the accurate placement of the bladder directly over the artery (this is true even with the use of microphones in lieu of a stethoscope). Secondly, known electronic blood pressure measuring devices rely entirely on pickup of the characteristic changes in Korotkoff (pressure wave) sounds and thus employ conventional microphones. The need to filter and isolate the correct sounds results in relatively complex and thus costly circuitry. Furthermore, the number of circuit components required is large and this has an effect on labor costs and system reliability. A third shortcoming is that each device is designed for only taking the blood pressure of humans and for using, for example, the left arm of the patient for the measurement. It is a recognized rule that in order to obtain an accurate reading, the width of the inflatable bladder should be approximately 20% greater than the diameter of the limb on which it is used. Therefore, it would be an advantage if blood pressure devices could be provided with detachable and interchangeable cuffs so that the blood pressure of different sizes of patients, and especially infants, could be accurately measured by using a single measurement unit, such as a sphygmomanometer, but with different, interchangeable, compression cuffs. Another shortcoming with conventional devices is that a portion of the medical personnel's attention is taken up with the activity of manually releasing the pressure from the bladder and attempting to do so at a regular rate, such as 2-3 mm of Hg per second. Automatic air pressure release means would permit the medical personnel to devote their full attention to hearing and noting the corresponding pressure at systolic and diastolic conditions and the monitoring of a dial to make such determinations.
A further shortcoming of such present-day devices involves the fact that these conventional devices are only usable on human beings and are not acceptable for animals due to the differences in anatomy between such animals and human beings. For example, in dogs, the arteries within the animal's limbs are somewhat recessed and a conventional cuff and bladder applies pressure to only one side of the limb and will act to push the artery back and away from the surface of the limb where the stethoscope bell is normally placed. The surface of the artery will thus not be in close proximity to the bladder portion of the cuff as is required for accurate readings. A further difficulty with animals, such as dogs, is that the animal's artery in an appendage, such as a hind leg, may be somewhat randomly positioned and thus may be difficult to locate and the requisite precise placement of the stethoscope bell over the artery may not be possible. In addition, when the size of the animal's appendage is small, there may not be sufficient surface skin area to permit the bell of the stethoscope to make a seal over the artery and due to the fact that the edges of the stethoscope overhang the sides of the appendage, the accuracy of any readings taken will be subject to noticeable error. Therefore, arterial catheterization--a method which is both lengthy and complicated--must be used on animals in order to accurately measure their blood pressure.
Although only two classes of subjects have heretofore been mentioned, human beings and animals, a third class exists. This class includes infants whose physical size requires a specially sized compression cuff and whose artery size and position make conventional devices unreliable. Infants' arteries are so small, such as the brachial artery which is often used, that they are difficult to find and when a stethoscope must be used (as with conventional blood pressure measuring devices) positioning variations can cause a 10 to 20 percent difference in the readings. Also, as previously mentioned, there may not be adequate skin area over the artery for the bell of the stethoscope to make an air-tight seal. Consequently when an infants' condition is being monitored, there is no way to tell if blood pressure variations are due to the measuring equipment and technique or due to a physical change in the infant. This problem can be eliminated by providing measurement means which does not require a stethoscope or the precise positioning of pick-up microphones. It would also be an improvement to such devices if other useful data could be obtained at the same time blood pressure readings are taken, and displayed in a conveniently readable manner, without having to subject the patient to further medical procedures or testing.